With the development of better and better barrier materials, generally from composites that include plastic films, it has now become very desirable to be able to precisely measure the rate of permeation through such barrier materials in order to properly evaluate them. As barrier materials have improved in their resistance to moisture and oxygen permeation, it has become clear that better, more sophisticated methods and apparatus are required to be able to accurately measure such lower and lower rates of permeation representative of barrier materials presently needed for commercial applications.
Gas permeability measuring devices have generally been known in the art, and some of these have been developed to serve the garment industry where fabrics that are highly resistant to water permeation are often desired. However, more recently, with the development of LCD's, LED's and OLED's, it has become important to develop barrier materials that have an extremely high resistance to moisture permeation and oxygen permeation. Although it has been shown scientifically that there is a relationship between the permeation of moisture and the permeation of oxygen through a barrier, so that by measuring the moisture permeation rate, an assessment can be obtained for the resistance of a barrier film or the like to the permeation of both moisture and oxygen, more precise measurements of oxygen permeation are often desired.
Because many present day products have been found to be highly sensitive to oxygen and moisture, often resulting in significant deterioration of the product, there has been a recent emphasis on developing better barrier materials. Products in the electronics fields, such as OLED's and LCD's, and certain pharmaceuticals are among products for which it is most important to resist such deterioration. The barrier materials that have been developed to protect such materials generally include multilayer composites made of polymeric films and thin layer inorganic materials, and the search has gone on for providing increasingly better multilayer, thin film barrier materials for this purpose. For example U.S. Pat. No. 6,413,645 entitled “Ultrabarrier Substrates” describes the problem and the search for more permeation-resistant materials. However, this patent states that oxygen and water vapor transmission rates even as high as 0.005 cc or gm/m2/day are below the detection limit of current industrial instrumentation.
To measure moisture permeation, efforts have been made to use the amount of change in weight of a suitable desiccant in a closed container where the object closing the container has its opposite face exposed to a humid atmosphere; however, the accuracy such apparatus has been frequently called into question. U.S. Pat. No. 3,580,067 illustrates another effort to use a desiccant in a closed container which is then placed in a chamber wherein a humidity of tritiated water vapor is maintained. The procedure involves batch analysis/counting where a plurality of sealed vials are needed, and the technician performing the procedure would be exposed to radioactive and toxic tritiated water vapor, each time a vial is removed and then handled. One must add a counting solution to dissolve the desiccant and facilitate counting on a liquid scintillation counter; the results are compared with a blank vial, containing the same amount of desiccant and counting solution. This provides a single point on a graph, and multiple points must be plotted before a measured water vapor transmission rate can be obtained. U.S. Pat. No. 4,663,969, issued May 12, 1987, discloses apparatus for testing water vapor transmission which employs a heated water bath; a solution containing a solute is employed along with an electrical conductivity measuring device to measure the change in concentration, which will be indicative of moisture permeation. However, it is felt that such an apparatus is not suitable to measure extremely low rates of moisture permeation.
U.S. Pat. No. 6,119,506 discloses an apparatus that is designed to allow measurement of mass transport. The flux of water vapor through a film or other object being measured is calculated by measuring results for exposure to a dry gas atmosphere, to a water-saturated atmosphere, and to atmospheres of different relative humidities; a computer program is used to determine transmission rate through the object being tested. Humidity probes are used to provide outlet signals that are indicative of the water vapor concentration in nitrogen streams that are flowed through a cell where such testing is occurring. In addition to being somewhat complicated, the apparatus is not felt to be well-suited to measuring extremely low moisture diffusion rates.
An instrument for determining the permeation rate of a gas, such as carbon dioxide, through a membrane is disclosed in U.S. Pat. No. 3,590,634, issued Jul. 6, 1971. The instrument flows a gas stream of constant concentration through an upstream chamber while flowing a carrier gas stream, such as helium, through a downstream chamber in order to measure permeation through a film which separates the two chambers. A thermal conductivity cell is provided to measure this characteristic of the carrier gas stream exiting at the downstream chamber, and an involved calibration system is required to interpret the data by comparing it with calibrations using a helium-carbon dioxide mixture of known composition. The arrangement is admittedly inappropriate for calculating very low permeation rates; it is stated that, when a continuous signal of noticeable magnitude is not being produced, it is necessary to isolate the permeation cell (by either halting or diverting the flow of the carrier gas) for a fixed time interval during which vapor is allowed to accumulate in the downstream chamber. After some predetermined time period, the accumulated vapor is allowed to be carried to the chamber, and the recorded peak and the area under the peak must then be graphically analyzed. The arrangement is clearly unsuitable for measuring ultralow permeation rates of oxygen.
In order to be able to effectively evaluate the performance of these new materials and other situations where highly effective barriers are involved, adequate test equipment is required for detecting moisture and/or oxygen permeation at these extremely low levels. Thus, more accurate apparatus and methods have been sought for measurement of such ultralow permeation rates.